Calcium carbonate is widely used as a filler or pigment for rubbers, plastics, paints, paper, cosmetics and so forth. Spherical calcium carbonate, in particular, has various superior characteristics such as filling property, dispersibility and lubricating property, and is used for mat coated paper in the field of papermaking and, as the other fields, cosmetics and so forth. As spherical calcium carbonate used for these uses, one having a small particle size (for example, several micrometers or smaller) has been conventionally desired in order to obtain a high degree of whiteness, gloss and dispersibility. However, in recent years, a pigment having a large particle size has come to be desired with multi-functionalization of the products for which calcium carbonate is used. For example, in the field of papermaking, a pigment having a mean particle diameter of 10 μm or larger is used for lusterless mat coated paper having a low degree of gloss. Moreover, also in cosmetics, a pigment having a specific feeling of touch, which cannot enter into pores, has comes to be used.
As the method of producing spherical calcium carbonate, various methods have been proposed so far, including a method of reacting an aqueous calcium chloride solution with an aqueous sodium carbonate solution, a method of reacting a water-soluble calcium salt with a carbonate salt in an aqueous solution in the presence of divalent cations other than calcium cations, a method of reacting calcium chloride and a hydrogencarbonate by using a phosphoric acid compound to produce vaterite type spherical calcium carbonate, and so forth. The method of using a phosphoric acid compound is described in, for example, Japanese Patent Unexamined Publication (KOKAI) No. 6-16417.
Further, methods of producing spherical calcium carbonate by using an additive when calcium carbonate is produced by introducing a carbon dioxide type gas into calcium hydroxide slurry have also been proposed. For example, Japanese Patent Publication (KOKOKU) No. 4-4247 describes a method of producing precipitated spherical calcite having a mean particle diameter of 2 to 10 μm by introducing carbon dioxide into calcium hydroxide slurry having a concentration of 15 to 20% which a predetermined amount of a polyphosphoric acid salt is added. Further, Japanese Patent Publication (KOKOKU) No. 7-33433 describes a method of producing calcite type spherical calcium carbonate having a mean particle diameter of 0.1 to 1.5 μm by mixing a reaction mixture in which calcium hydroxide slurry and carbon dioxide are reacted, and when the conductivity of the reaction mixture reaches a predetermined level, an oxyacid salt of phosphorus or a salt of a polymer or copolymer of unsaturated carboxylic acid is added and reacted, and a reaction mixture in which calcium hydroxide slurry and carbon dioxide are reacted until the conductivity of the reaction mixture reaches a predetermined level.
However, all the types of spherical calcium carbonate obtained by these conventional methods for producing spherical calcium carbonate have a mean particle diameter of less than 10 μm, and such spherical calcium carbonate having a comparatively large mean particle diameter as mentioned above cannot be produced. Although Japanese Patent Unexamined Publication No. 11-79740 proposes a method of producing spherical calcium carbonate secondary particles having a large specific surface area by spray-drying calcium carbonate obtained by introducing a carbon dioxide type gas into calcium hydroxide slurry, the spherical calcium carbonate obtained by this method consists of secondary particles having a mean particle diameter of 45 to 75 μm.
Therefore, an object of the present invention is to provide calcite type spherical calcium carbonate having a mean particle diameter of 10 μm or larger, showing high degree of whiteness and small friction coefficient as physical properties of fine particles, and having a shape comparatively close to a true sphere. Another object of the present invention is to provide utilization of such spherical calcium carbonate for various uses.